Mechanical refrigeration is primarily an application of thermodynamics wherein a cooling medium, such as a refrigerant, goes through a cycle so that it can be recovered for reuse. Commonly used cycles include vapor-compression, absorption, stem-jet or stem-ejector, and air.
There are various types of compressors that may be used in refrigeration applications. Compressors can be generally classified as reciprocating, rotary, jet, centrifugal, or axial-flow, depending on the mechanical means to compress the fluid, or as positive-displacement or dynamic, depending on how the mechanical elements act on the fluid to be compressed.
A centrifugal compressor uses rotating elements to accelerate the refrigerant radially, and typically includes an impeller and diffuser housed in a casing. Centrifugal compressors usually take fluid in at an impeller eye, or central inlet of a circulating impeller, and accelerate it radially outwardly. Some static pressure rise occurs in the impeller, but most of the pressure rise occurs in the diffuser section of the casing, where velocity is converted to static pressure. Each impeller-diffuser set is a stage of the compressor. Centrifugal compressors are built with from 1 to 12 or more stages, depending on the final pressure desired and the volume of refrigerant to be handled.
The pressure ratio, or compression ratio, of a compressor is the ratio of absolute discharge pressure to the absolute inlet pressure. Pressure delivered by a centrifugal compressor is practically constant over a relatively wide range of capacities.
Unlike a positive displacement compressor, a centrifugal compressor depends entirely on the centrifugal force of the high speed impeller to compress the vapor passing through the impeller. There is no positive displacement, but rather what is called dynamic-compression.
The pressure a centrifugal compressor can develop depends on the tip speed of the impeller. Tip speed is the speed of the impeller measured at its tip and is related to the diameter of the impeller and its revolutions per minute. The capacity of the centrifugal compressor is determined by the size of the passages through the impeller. This makes the size of the compressor more dependent on the pressure required than the capacity.
Because of its high speed operation, a centrifugal compressor is fundamentally a high volume, low pressure machine. A centrifugal compressor works best with a low pressure refrigerant, such as trichlorofluoromethane (CFC-11). When CFC-11 is used as the refrigerant, suction pressure in the compressor is from about 18 to 25 inches of vacuum depending on the evaporator temperature required, and the discharge pressure is near atmospheric pressure. A single stage impeller can be used with CFC-11 for air conditioning suction temperatures.
A two-stage impeller is common for many conditions. In operation, the discharge of the first stage impeller goes to the suction intake of a second impeller. Each stage can build up a compression ratio of about 4 to 1, that is, the absolute discharge pressure can be 4 times the absolute suction pressure.
Centrifugal compressors range in size from 200 to 10,000 kilowatts of refrigeration capacity. For applications requiring more or less refrigeration capacity than CFC-11, 1,2,2-trichloro-trifluoroethane (CFC-113) or 1,2-dichlorotetrafluoroethane (CFC-114) can be used as the refrigerant in place of CFC-11 without changing the compressor except for providing a properly-sized motor.
A proposed world-wide reduction in the production of fully halogenated chlorofluorocarbons such as CFC-11, CFC-113, and CFC-114, has developed an urgent need for alternative, more environmentally acceptable products.
Large investments have been made in centrifugal compressors that were designed for CFC-11, CFC-113, or CFC-114. A centrifugal compressor is designed for the refrigerant with which it is to be used. That is, a centrifugal compressor is typically designed by first selecting a refrigerant, and then determining the desired refrigeration capacity and power source. Once these variables are known, the diameter of the impeller, the size of the impeller opening, and the number of stages are designed to achieve the desired refrigeration capacity.
A problem with replacing chloroflurocarbons with alternative refrigerants for use in existing centrifugal compressors that unless the alternative refrigerant matches certain physical criteria, the alternative refrigerant will not work in the existing centrifugal compressor. Important criteria include the "tip speed" of a refrigerant, meaning the speed of the impeller as measured at its tip for a given centrifugal compressor, and the density and molecular weight of the refrigerant.
If it is desired to replace a refrigerant in a centrifugal compressor, and the replacement refrigerant does not perform as well the original refrigerant, it is possible to design a compressor for the replacement refrigerant and to replace the original compressor. However, replacing an existing compressor is not possible in all cases. For example, a centrifugal compressor may be so large (such as is used in the cooling system of large buildings) that it cannot be replaced by a redesigned compressor. In such cases, the replacement refrigerant must work in the original compressor.